If you drop down to a 3.7 it will get you around 3100-3200 rpm. Honestly a 3.25 would be a cruiser final gear setup but I would still give it a shot.
Email these guys and see if they have any 3.7 or...

If you installed a 3.88 and you are at 4000rpm at 70mph, then i don't believe you got a 3.88 final gear set. With the crappi.e.st 5th gear of .771, a final diff of 3.88, and 10" tires, you should be...